This invention relates to improvements in the control of heating devices. It is particularly concerned with the control of a heater for a railway track switch.
In my U.S. Pat. No. 3,972,497, issued Aug. 3, 1976, I describe apparatus for producing snow deflecting air currents for a railway switch. As stated therein, in the winter operation of railway systems the failure of railway track switches due to the presence of snow is a well known problem. Railway track switches are presently protected against failure from snow or ice by manual cleaning and by thermal methods, e.g. electrical heating, and by combustion heating.
Many railway switches use oil or gas fired burners to provide heated air for maintaining the switch points clear of snow and ice. The burners may be controlled locally by a snow detector or remotely by a dispatcher often located many miles from the site of the switch where environmental conditions may be very different.
In. U.S. Pat. No. 3,439,161, issued Apr. 15, 1969 to L. A. McElwee et al, a railway switch heater is described together with a rail temperature sensor to provide an indication of the temperature of the rails which indication may be so used that when the rails are sufficiently heated assuring proper operation of the switch, the heater may be shut off.
Experiments with a rail temperature sensor have indicated that control is not adequate when one is concerned with the snow and ice problems which are normally encountered during winter operation of a railway. Furthermore, reliance on the temperature of a rail alone may well result in the heater being switched on when there is no accumulation of snow and ice to prevent operation of the railway switch. Thus an appreciable wastage of fuel may result.